diesel engine injectors
Appointment of nozzles and requirements for them
The nozzle is used to supply fuel to the engine cylinder, spray and distribute fuel through the combustion chambers.
The operating conditions of the injectors are very difficult - they are exposed to enormous pressures and thermal loads. Injection starts at combustion chamber temperature 700…900 ˚С and pressure 3…6 MPa, and ends at temperatures up to 2000 ˚С and pressure 10…11 MPa.
The following very stringent requirements are imposed on the nozzles:
- optimal dispersity, i.e., a high degree of crushing of fuel droplets, since the smaller the droplet, the greater their total surface, the fuel is heated and burned faster, but the flame length decreases;
- ensuring that the speed of the fuel jet reaches the edges of the combustion chamber, so the droplets should not be too small - average droplet size (taking into account the requirement in the first paragraph) - 30…50 µm;
- distribution of injected fuel throughout the volume of the combustion chamber;
- sharp start of injection and its termination.
Nozzles are open And closed.
Open nozzles provide a constant supply of fuel. In modern diesel engines, such nozzles are not used.
In diesel engines, closed nozzles are used, which open only when fuel is supplied to the combustion chamber.
Closed nozzles can be of two types − single and multi-hole. The former are installed on engines with, the latter with undivided combustion chambers.
There are also mechanical nozzles and electronically controlled nozzles.
Modern diesel engine power systems use computer-controlled injection (electronic control unit). Based on information from numerous sensors, such systems take into account many processes and current parameters of the engine. The nozzles in such systems are controlled by special electromagnetic or piezoelectric devices, which opens up wide opportunities for improving the efficiency of the engine, as well as its environmental friendliness.
The history of the invention of the nozzle
As you know, Rudolf Diesel initially planned the work of his famous brainchild on coal dust. Its power system contained a special pump that blows coal dust into the engine cylinder with compressed air. However, coal turned out to be a low-calorie fuel, unable to give a high combustion temperature, and Diesel had to turn his ingenious eye to liquid fuels. After all, the temperature difference in the engine cycle is a direct way to increase efficiency, as the Frenchman established.
At first, Diesel tried to inject gasoline into the cylinder of his engine, but at the very first test of the engine, an explosion occurred that almost cost the life of Diesel himself and his assistants, and the inventor had to use a less explosive fuel - kerosene.
In June 1894, Diesel built an engine that used kerosene as fuel, which was injected into the cylinders with a special nozzle. For the injection of kerosene, a pneumatic compressor was used, which developed a pressure exceeding the pressure in the engine cylinder. The name "compressor diesels" was assigned to such engines.
The idea of hydraulic fuel injection in diesel engines belongs, according to history, to the French engineer Sabate, who, moreover, proposed multiple injection, that is, injection carried out in several stages (this idea is used in modern power systems - Common Rail and pump -nozzle).
In 1899, the Russian engineer Arshaulov built and introduced for the first time a high-pressure fuel pump of an original design - driven by air compressed in the cylinder, working with a compressorless nozzle. These nozzles were installed on diesel engines produced by the Ludwig Nobel Mechanical Plant in St. Petersburg at the beginning of the last century (“Russian diesel engines”).
In the 20s of the XX century, the German engineer Robert Bosch improved the built-in high-pressure fuel pump, and also created a successful modification of the compressorless injector. These devices, with various improvements, are used in diesel engine power systems to this day.
Diesel engines that use an increase in fuel pressure in the fuel system before injection are called “compressorless diesel engines”.
At present, classical compressor diesel engines have no practical application. In modern engines, injection is carried out by non-compressor methods.
However, science and technology do not stand still, and, thanks to the widespread computerization of all vehicle systems, mechanical injectors are now gradually being replaced by more advanced electronically controlled devices.
The principle of operation of the multi-hole nozzle
In a multi-hole nozzle, the main part is the atomizer. It consists of a body 1
(rice. 1, a) and needles 2
. The atomizer is drawn to the body 7
nozzles with union nut 3
. A spring presses on top of the needle 12
(rice. 1, b). Fuel into the cavity B nozzles are fed through the channel IN.
When there is no fuel supply by the pump ( rice. 1. I), pressure in the cavity B is 2…4 MPa. Fuel presses on the load belt G needle, but this force is less than the spring force that presses the needle against the atomizer. Needle locking cone D covers outlets - nozzle A.
When fuel is supplied by the pump, the force of fuel pressure on the girdle G becomes more spring force, the needle rises, and through the nozzle A fuel is injected into the combustion chamber at high speed. After the end of the fuel supply, the pressure drops, the spring returns the needle to its place, blocking the nozzle outlets, and the injection stops.
The rise of the needle is limited by the emphasis of its upper shoulders in the body 5 injectors and makes up 0.2…0.25 mm.
The quality of fuel crushing depends on the speed of its movement through the nozzles, which, in turn, depends on the injection pressure. In normal mode, the fuel jet velocity is 200…400 m/s. To do this, it is necessary to create a pressure difference in the nozzle and combustion chamber 5…10 MPa. Since the pressure in the cylinder at the moment of injection reaches 3…5 MPa, the fuel pressure in the injector should be more than 10…20 MPa.
To ensure the operation of the nozzle at this pressure, the body of the atomizer and the needle are made very accurately and lapped to each other. They are the third precision pair in the high pressure line. The needle and the body of the atomizer cannot be dismantled and must be replaced only as a set.
Multi-hole nozzle device
On engines with undivided combustion chambers, as a rule, multi-hole nozzles are installed. So, on KamAZ-740 engines, nozzles of the 33 series are installed, on ZIL-645 and YaMZ-240 engines - B-2SB nozzles, on YaMZ-238 engines - model 80 nozzles ( see figure 2 at the bottom of the page).
To the body 7 nozzles with union nut 3 attached atomizer with a needle 2 . The atomizer has four nozzle holes with a diameter 0.3mm. On the needle through the bar 13 spring presses 12 . Fuel from the pump is supplied to the nozzle cavity through the fitting 9 in which the filter is installed 10 . The upper opening in the housing serves to drain fuel that has leaked through the gaps between the needle and the sprayer into the tank. Pins 4 And 6 determine the exact position of the atomizer relative to the housing and fuel channels. Gaskets 11 adjust the spring tension, which determines the injection start pressure.
The injectors are installed in special sockets in the cylinder head and secured with brackets.
A sealing copper washer (ring) is placed between the nozzle body and the block head, which is put on the atomizer body and, together with the nozzle, is carefully inserted into the head socket. Such a washer serves not only as a seal between the nozzle and the head, but also provides good heat dissipation from the atomizer to the cylinder head.
O-ring 8 protects the cavity of the valve cover from dust and moisture.
The device of a single-hole pin nozzle
Single-hole nozzles are sometimes called pin nozzles, since the end of its needle is made in the form of a pin. Such nozzles are installed, as a rule, in diesel engines with separated combustion chambers.
The design of the atomizer of such injectors provides, since the atomization of fuel is more directed than in multi-hole injectors, and a significant part of the fuel reaches the walls of the combustion chambers, forming a rapidly evaporating film.
Diesel engines with vortex (separate) combustion chambers are less sensitive to fuel composition and operate more stably in a wide range of rotational speeds. The nozzles used with them are designed for lower pressure, therefore, they do not require such high manufacturing precision as nozzles for undivided combustion chambers, and therefore are cheaper.
On rice. 1, in the atomizer of a single-hole pin nozzle is shown. Such a nozzle is installed in vortex combustion chambers and has one nozzle.
Needle end 2
made in the form of a pin 13
conical shape protruding beyond the body of the atomizer. The pin serves to form a fuel jet in the form of a cone.
The principle of operation of single-hole nozzles does not differ from the principle of operation of multi-hole nozzles.
The device of some types of injectors used on domestic autotractor diesel engines is shown in figure 2.
